1. Field of the Invention
The invention relates to fluid-flow turbines, such as wind turbines under water current turbines, and to other prime movers, and more particularly to variable speed turbines employing multi-phase generators with power conversion technology for torque control and rotor blade pitch for turbine speed and load control.
2. Description of the Prior Art
The development of practical, wind-powered generating systems creates problems, which are unique and not encountered in the development of conventional power generating systems. These problems are similar in nature to under water current turbines, mining equipment and wind tunnel boring equipment. The natural variability of the wind affects the nature and quality of the electricity produced and the relationship between the velocity of the tip of a turbine blade and the wind velocity affects the maximum energy that may be captured from the wind. These issues together with mechanical fatigue due to wind variability have a significant impact on the cost of wind generated electricity.
Historically, wind turbines have been operated at constant speed. The power delivered by such a wind turbine is determined by the torque produced by blades and main shaft. The turbine is typically controlled by a power command signal, which is fed to a turbine blade pitch angle servo, referred herein as a Pitch Control Unit or PCU. This servo controls the pitch of the rotor blades and therefore the power output of the wind turbine. Because of stability considerations, this control loop must be operated with a limited bandwidth and, thus, is not capable of responding adequately to wind gusts. In this condition, main-shaft torque goes up and transient power surges occur. These power surges not only affect the quality of the electrical power produced, but they create significant mechanical loads on the wind turbine itself. These mechanical loads further force the capital cost of turbines up because the turbine structures must be designed to withstand these loads over long periods of time, in some cases 20–30 years.
To alleviate the problems of power surges and mechanical loads with constant speed wind turbines, the Wind Power industry has been moving towards the use of variable speed wind turbines. A variable speed wind turbine employs a converter between the generator and the grid. Because the turbine generator is now decoupled from the grid, the frequency and voltage at which the generator operates is independent of the constant voltage, constant frequency of the grid. This permits variable speed operation. Two classes of power converter have been employed in this application. The first is referred to as a full conversion system, which is inserted between the generator and grid as described. In this approach, the converter carries all of the generated power. An example of this type of system is described in U.S. Pat. No. 5,083,039, entitled “Variable Speed Wind Turbine”, issued Jan. 21, 1992. In the second class, the converter is placed between a portion of the generator and the grid, usually the rotor circuit. This approach is used because the converter only needs to be sized for a portion of the total power. This is referred to as partial conversion and an example of this approach is described in U.S. Pat. No. 6,137,187, U.S. Pat. No. 6,420,795 and U.S. Pat. No. 6,600,240 all entitled “Variable Speed Wind Turbine Generator”.
The variable speed wind turbine disclosed in U.S. Pat. No. 5,083,039 comprises a turbine rotor that drives a pair of AC squirrel cage induction generators with two respective power converters. The converters contain an active rectifier that controls the generator torque by means of a high-performance field-orientation method. The converter also contains an inverter section, which is synchronized to the AC line and controls the DC bus voltage by maintaining a power balance between the generator and the AC grid. The converter is inherently bi-directional and can pass power in both directions. The inverter section of the converter is capable of shifting the current waveform relative to the grid voltage and variable reactive power, or power factor can be controlled in this way. With an induction generator, this system requires an active rectifier as the magnetizing component of the generator must be supplied by the DC bus through proper control of the active rectifier.
U.S. Pat. Nos. 6,137,187, 6,420,795, and 6,600,240 describe a partial conversion variable speed system for use in wind turbines. The system comprises a wound rotor induction generator, a torque controller and a proportional, integral derivative (PID) pitch controller. The torque controller controls generator torque using field-oriented control (on the rotor) and the PID controller performs pitch regulation based on generator rotor speed. Like the U.S. Pat. No. 5,083,039 patent, power flow is bi-directional within the rotor of the generator and an active rectifier and grid inverter is used for the conversion process. The converter used in this system is rated at only a portion of the total turbine rating, with the rating depending on the maximum generator slip used in the turbine design. The converter controls the current and frequency in the rotor circuit only with a direct grid connection to the utility. Because the generator operates at sub-synchronous and super-synchronous speeds, the converter must also be bi-directional just as in the U.S. Pat. No. 5,083,039 case. In addition to the converter controlling torque in this system, the converter is capable of controlling system reactive power or power factor. This is accomplished by under/over exciting the generator rotor circuit along its magnetization axis. The converter is placed off line from the stator connection to the grid and only handles rotor power input and output. The control of the pitch system is also covered in this patent. The pitch system simply responds to a speed error through a proportional, integral, derivative controller (PID) to call for the correct pitch angle to maintain speed. A further advantage of variable speed wind turbines is that through the use of their solid-state power conversion technology, utility interconnection power quality requirements have been improved beyond that achievable with constant speed wind turbines. Variable speed turbines have inherently better power regulation qualities resulting in less line voltage flicker. This allows these machines to meet demanding power quality standards such as IEEE 519.
By properly controlling the torque and pitch on the variable speed turbine, an increase in energy capture and load reduction is possible. This, together with the improved power quality, makes the variable speed turbine economically attractive for electrical power generation.